2 Answers
2

Depends on what you're doing with them. Shallots have very fine layers (allowing them to blend into sauces and dressings), and have a flavor between garlic and onions.

If you're mincing them fine and sauteeing them or whisking them in as the foundation for a sauce, I'd suggest substituting pearl (boiling) onions or the white parts of green onions and maybe half a clove of minced garlic.

If you're slicing and frying them, regular onions should do, although I'd look for smaller ones and slice thin. Red onions have a flavor more like shallots than yellow or white ones, but may add an undesireable color to the dish.

If you're chopping them, or cooking them peeled and whole for Southeast Asian cuisine, again pearl onions are probably the way to go.